From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from plane.gmane.org ([80.91.229.3]:40558 "EHLO plane.gmane.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751988AbbJEInl (ORCPT ); Mon, 5 Oct 2015 04:43:41 -0400 Received: from list by plane.gmane.org with local (Exim 4.69) (envelope-from ) id 1Zj1Mu-0002n7-KB for linux-btrfs@vger.kernel.org; Mon, 05 Oct 2015 10:43:36 +0200 Received: from coffee.modeemi.fi ([130.230.72.140]) by main.gmane.org with esmtp (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Mon, 05 Oct 2015 10:43:36 +0200 Received: from flux-btrfs by coffee.modeemi.fi with local (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Mon, 05 Oct 2015 10:43:36 +0200 To: linux-btrfs@vger.kernel.org From: Erkki Seppala Subject: Re: BTRFS as image store for KVM? Date: Mon, 05 Oct 2015 11:43:18 +0300 Message-ID: References: <55F88ECC.1040604@menke.ac> <201510021421.09558.russell@coker.com.au> <560E737C.7050206@gmail.com> <201510031832.17802.russell@coker.com.au> <56111598.2080403@bouton.name> Mime-Version: 1.0 Content-Type: text/plain Sender: linux-btrfs-owner@vger.kernel.org List-ID: Lionel Bouton writes: > 1/ AFAIK the kernel md RAID1 code behaves the same (last time I checked > you need 2 processes to read from 2 devices at once) and I've never seen > anyone arguing that the current md code is unstable. This indeed seems to be the case on my MD RAID1 HDD. But on MD SSD RAID10 it does use all the four devices (using dd on the md raid device and inspection iostat at the samy time). So the lack of MD RAID1 doing it on HDDs seems to be for devices that don't perform well in random access scenarios, as you mentioned. In practical terms I seem to be getting about 1.1 GB/s from the 4 SSDs with 'dd', whereas I seem to be getting ~650 MB/s when I dd from two fastest components of the MD device at the same time. As it seems that I get 330 M/s from two of the SSDs and 150M/s from the other two, it seems the concurrent RAID10 IO is scaling linearly. (In fact maybe I should look into why the two devices are getting lower speeds overall - they used to be fast.) I didn't calculate how large the linearly transferred chunks would need to be to overcome the seek altency. Probably quite large. -- _____________________________________________________________________ / __// /__ ____ __ http://www.modeemi.fi/~flux/\ \ / /_ / // // /\ \/ / \ / /_/ /_/ \___/ /_/\_\@modeemi.fi \/